The invention relates to electric circuits for equalizing cell voltages within a battery of cells.
Many products employ a battery of cells as a power source and a great deal of research effort is directed towards designing cells which provide the best possible performance characteristics. Characteristics of interest include cell voltage, cell capacity, and for re-chargeable batteries, cell charge-discharge cycle lifetime and cell behavior during charging and discharging.
For the various types of cell currently available, individual voltages are typically on the order of a few volts. However, many applications require higher voltages and so use a battery of cells connected in series to provide an overall battery voltage equal to the sum of the cell voltages in the battery.
FIG. 1 is a schematic diagram showing a battery 1 comprising eight serially-connected rechargeable lithium-ion (li-ion) cells 2A-H. The battery 1 is connected to a load 4, in this example a motor, via a load-switch 6. The battery 1 is also connected to a charger 8 via a charger-switch 10. The eight cells 2A-H respectively provide cell voltages VDCA-H. The battery voltage (i.e. the voltage difference between a battery cathode 12 and a battery anode 14) is VDCbatt and is equal to the sum of the individual cell voltages VDCA-H. Each of the cells 2A-H provides a nominal cell voltage of 4V and the battery 1 accordingly provides a nominal battery voltage of 32.0V.
When the battery is in use the load-switch 6 is closed and the charger-switch 10 is open (as shown in FIG. 1). In this switch configuration, the battery voltage is applied to the load and the load draws a current IL from the battery, no current is supplied by the charger 8. When the cells are supplying current in this manner, they are said to be discharging.
When the battery is being charged the load-switch 6 is open and the charger-switch 10 is closed. In this switch configuration, the charger 8 supplies a current IC to the battery for recharging the cells 2A-H whilst the load draws no current from the battery. When the cells receive current in this manner they are said to be charging.
Since the cells 2A-H in the battery 1 are connected in series, Kirchoff's current flow law ensures that the current IL (IC) passes equally through each of the cells during discharging (charging). For example, if during discharge the battery supplies 1 amp for 1 second (so passing a total charge of 1 coulomb through the load), the capacity of each individual cell is reduced by 1 coulomb, irrespective of the individual cell voltages VDCA-H. Similarly, if the battery 1 receives 1 amp for 1 second from the charger, the capacity of each individual cell is increased by 1 coulomb.
The discharging and charging characteristics of even nominally identical cells will generally be different. Differences in internal geometry and chemical composition will occur even in cells manufactured within a single batch. The cells 2A-H in the battery 1 will typically have different total capacities and different rates of voltage decay (increase) during discharging (charging). It is known that for a cell to provide optimum performance parameters, for example maximum capacity, high charge/discharge cycle lifetime etc., it should generally not be charged above an over-charge threshold or discharged below a deep-discharge threshold. Differences in the charging and discharging characteristics of cells in a battery make it difficult to properly manage the charging and discharging of individual cell, this can significantly reduce the overall battery performance.
As an example of problems which can arise during discharging, suppose cell 2A in the battery shown in FIG. 1 has a lower capacity than the cells 2B-H (which are identical) such that VDCA falls faster than VDCB-H. All cells are initially charged to 4.0V to provide an initial battery voltage of 32V (although the load can operate with a battery voltage as low as 24V). After a period of discharge, the voltages of cells 2B-H falls to 3.5V, whilst in the same period the voltage of cell 2A falls to 3V. The battery voltage is now 27.5V. If 3.0V represents the deep-discharge threshold, further use of the battery will have a deleterious effect on cell 2A. However, if discharge is stopped to protect cell 2A, the remaining capacity of cells 2B-H is unused, and the battery requires recharging before its total useful capacity has been exhausted. Furthermore, if individual cells are to be protected, a voltage monitor circuit is required for each cell.
As an example of problems which can arise during charging, again suppose cell 2A has a lower capacity than the cells 2B-H (which again are identical) such that VDCA rises faster than VDCB-H during charging. All cells are initially at 3V, but after a period of charging, the voltage of cells 2B-H rises to 3.6V but in the same period the voltage of cell 2A rises to 4.2V. At this point, the battery voltage is 29.2V. If 4.2V represents the over-charge threshold, further charging of the battery will have a deleterious effect on cell 2A. However, if charging is stopped to protect cell 2A from over-charge, the battery voltage is limited to 29.2V, so reducing its capacity during subsequent use. If each cell is to be individually protected, a voltage monitor circuit is again required for each cell.